The present invention relates to an injection nozzle and in particular to an injection nozzle for mixing heads of reaction molding machines, wherein a nozzle needle is adjustable for precise operation of the injection nozzle.
When producing form parts according to the reaction injection molding method, two or more liquid, reactive compounds are mixed together whereby a chemical reaction is initiated resulting in a plastic material, which hardens into a form part while in a form tool.
Frequently used in the production of polyurethane-form parts are the compounds polyol and isocyanate. Production of such form parts may be carried out by means of high pressure-injection mixtures, where the compounds are preferably injected by means of injection nozzles that are preferably directed towards each other into the so-called mixing chamber where they mix together as a result of the kinetic energy, but also through the turbulences that are created within the small mixing space. Various mixing heads are known in the prior art (Johannaber, Plastic Machinery Guide, 3. Edition, 1992 pp. 567-571). These mixing heads all have in common the use of injection nozzles for supplying the compounds into the mixing chamber.
When adjusting the nozzle pressure, respectively the injection pressure, it is known in the prior art to adjust the position of the nozzle needle and the resulting nozzle gap, by means of a spring element. By means of the spring, an adjustable force can be imparted onto the nozzle needle when the spring is pre-tensioned, which thereby counteracts the pressure of the compounds that are supplied via this injection nozzle.
In some cases, it is necessary or desirable during production of form pieces by the injection molding technique, to adjust the metering amount of one or both reaction compounds from one form piece to another form piece, or perhaps even to make an adjustment during the production of one form piece. For example, when producing first a small form piece with small discharge capacity and subsequently a distinctly larger form piece, it is desirable, to produce the larger form piece with a larger discharge capacity, which requires corresponding enlargement of the nozzle gap. For a reduced discharge capacity, the nozzle gap is to be reduced correspondingly.
For the afore-described production purpose, an injection nozzle, which is spring loaded is for that purpose disadvantageous, since due to the characteristic behavior of the spring, an exactly reproducible injection pressure cannot be provided when producing varying form pieces. Accordingly, inaccurate injection pressures will have a negative impact on the quality of the form pieces.
A further disadvantage of the injection nozzle for mixing heads of the prior art arises is when the mixing head is opened, the nozzle gap is completely closed at the transition from the re-circulation condition to the mixing condition, which leads to pressure peaks at the compound feed side resulting in vibration in the spring loaded control system. A further disadvantage results, when due to the loss of friction in the spring itself, as well as between housing and the spring, which leads to a relatively large hysteresis in the control behavior. As a result, exact reproducibility of the nozzle pressures cannot be realized, since there is no way the gap sizes at the nozzle gap can be adjusted so they are fixed and reproducible.
From U.S. Pat. No. 4,378,335 a device for a reaction molding process is known, where the injection nozzles for the mixing heads are configured according to type of process used. Accordingly, a adjustment assembly is provided which is combined and unitary with a nozzle needle in the injection nozzle, and which is driven back and forth into different positions for adjustment of the injection gap.
From U.S. Pat. No. 5,443,797, a process and a device for reaction molding is known, in order to produce an improved mixing of the components, wherein the compound mixture, which is flowing into a nozzle area is divided into two streams that in turn stream around a piston and which then combine again at the other side of the separation point.